The compound (−)-picropodophyllin binds tightly to the human Insulin-like Growth Factor I Receptor Kinase (IGF1 RK) domain and inhibits its kinase activity. Such binding has the effect of inhibiting the anti-apoptotic signal pathways that are associated with IGF1 RK activity. This inhibition tends to decrease cancer cell growth and may also render cancer cells more susceptible to chemotherapy. Importantly, the compound, (−)-picropodophyllin, shows exquisite selectivity for the IGF1 RK over the structurally highly homologous Insulin Receptor Kinase (IRK) domain, the inhibition of which would give an undesired diabetes-type phenotype. This selectivity could lead to a new class of chemotherapeutic drugs.
At present, there is no drug on the market that acts upon the IGF1 RK. Development of such a drug is, therefore, an emerging macromolecular target of great current interest in the pharmaceutical community.
Human breast cancers have been molecularly classified by gene expression profiling into three major subtypes: luminal, ERBB2+ and basal-like. The luminal cancers are estrogen receptor-positive (ER+), whereas the cancers of the other two classes, which either overexpress ERBB2(HER2/NEU) or exhibit phenotypic features of basal/myoepithelial cells, are ER-negative (Perou et al., Nature (2000), 406, 747-52; Sorlie et al., Proc. Natl. Acad. Sci. U.S.A. (2001), 98, 10869-74; Sorlie et al., Proc. Natl. Acad. Sci. U.S.A. (2003), 100, 8418-23). Basal-like cancers also lack progesterone receptor (PR) and ERBB2 (ER−/PR−/ERBB2−; “triple negative breast cancers”), but frequently express EGFR and basal markers, such as cytokeratins 5/6 and/or 14 and p63 (Nielsen et al., Clin. Cancer Res. (2004), 10, 5367-74). The basal-like class (15-20% of all breast cancers; recently reviewed by Yehiely et al., Trends Mol. Med. (2006), 12, 537-44; Finnegan and Carey, Future Oncol. (2007), 3, 55-63; Da Silva et al., J. Clin. Pathol. (2007), 60, 1328-32) includes high proportions of BRCA1-associated and also medullary and metaplastic subtypes. The latter group is heterogeneous and consists of squamous and spindle cell carcinomas and other forms. Basal cancers appear to have extremely bad prognosis (Sorlie et al., Proc. Natl. Acad. Sci. U.S.A. (2001), 98, 10869-74), especially in the early years of follow-up after diagnosis and treatment, although this was not clearly evident in some patient cohorts (Chin et al., Cancer Cell (2006), 10, 529-41). In addition, these aggressive cancers pose a serious problem to targeted therapies, considering that the use of antiestrogens in combination with an anti-ERBB2 antibody (trastuzumab) is not an option, while there is no clear choice for chemotherapeutic intervention (see, for example, Cleator et al., Lancet Oncol. (2007), 8, 235-44; Carey et al, Clin. Cancer Res. (2007), 13, 2329-34).
Interestingly, an association between basal cancers and Kras amplification is now emerging. Ras-family members, apparently serving overlapping but also distinct cellular functions, act as molecular switches in signaling pathways regulating proliferation or apoptosis, and become constitutively active and, thus, oncogenic by mutation (usually at codons 12, 13 or 61). Although Kras is overall the most frequently mutated isoform, there is tissue-specificity and variable incidence of mutations in Ras tumorigenic action, as different human tumors harbor different mutant family members. In contrast to pancreatic ductal adenocarcinoma, in which Kras mutations (most commonly in codon 12) can be found at frequencies as high as 90% (Almoguera et al., Cell (1988), 53, 549-54), the incidence of Kras mutations in breast cancer appears to be low (˜7% on average; reported frequencies of 1/40, 1/25, 4/61 and 3/10; Rochlitz et al., Cancer Res. (1989), 49, 357-360; Prosperi et al., Cancer Lett. (1990), 51, 169-74) Myaikas, et al. Biochem. Biophys. Res. Commun (1998), 251, 609-612; Chen, et al., Cancer Lett., (2005), 229, 115-22). In human breast cancer cell lines, however, the observed frequency was higher (˜13%; 5/40; Hollestelle et al., Mol. Cancer. Res. (2007), 5, 195-201). Signaling elicited by non-mutated, but overexpressed Ras can also collaborate with other deregulated pathways in tumor progression and invasion (Clark and Der, Breast Cancer Res. Treat. (1995), 35, 133-44). For example, in comparison with control breast tissue, 11 of 20 breast cancers exhibited a 2- to 6-fold increase in enzymatically measured Ras activation (relative amount of GTP-bound form; von Lintig et al., Breast Cancer Res. Treat. (2000), 62, 51-62), while Western analysis indicated that in ˜70% of primary breast cancer specimens (n=132) the level of Ras was higher than in normal breast tissue (Dati et al., Int. J. Cancer (1991), 47, 833-38). Notably, in 56% ( 9/16) of examined basal-like human breast cancers identified by expression profiling, the Kras locus was amplified and, thus, overexpressed (Herschkowitz et al., Genome Biol. (2007), 8, R76).
The IGF signaling system, which is the major determinant of mammalian organismal growth (Efstratiadis A Int. J. Dev. Biol. (1998), 42:955-976), has also been implicated in the pathogenesis of various human cancers (Pollak M N, et al., Nat. Rev. Cancer (2004), 4:505-518.), including breast tumors (Sachdev D and Yee D, J Mammary Gland Biol Neoplasia (2006), 11:27-39). A seminal observation, in this regard, was that cells lacking Igf1r, the tyrosine kinase receptor mediating the effects of insulin-like growth factors, cannot be transformed by any one of several tested oncoproteins (Sell C, et al. Proc. Natl. Acad. Sci. USA (1993), 90:11217-11221; Sell C, et al. Mol. Cell. Biol. (1994), 14: 3604-3612; Baserga R, Expert Opin. Ther. Targets (2005), 9:753-768). Signaling through Igf1r does not appear to be an oncogenic component per se, but a crucial prerequisite for tumorigenesis because, among other actions such as the promotion of cellular proliferation by stimulation of the Ras/MAPK/ERK pathway, it exerts strong PI3 kinase-dependent and independent antiapoptotic effects that are necessary for tumor growth (Baserga R, Expert Opin. Ther. Targets (2005), 9:753-768). Moreover, the IGF system appears to be involved in resistance to certain anticancer regimes (Ryan P D and Goss P E, Oncologist (2008), 13:16-24). On the basis of these considerations, potential therapeutic approaches for cancer treatment involving blocking of IGF signaling with small molecules or antibodies are currently under development (Sachdev D and Yee D, J Mammary Gland Biol Neoplasia (2006), 11:27-39; Baserga R, Expert Opin. Ther. Targets (2005), 9:753-768; Ryan P D and Goss P E. Oncologist (2008), 13:16-24; Garcia-Echeverria C. IDrugs (2006), 9:415-419; Hartog H, et al., Eur. J. Cancer (2007) 43:1895-1904).